Electrostatic transducer having resilient electrode

ABSTRACT

An electro-acoustic transducer having a flexible diaphragm and a backplate of resilient construction and a front element of metalized foil. In a preferred embodiment of the invention the transducer is a condenser microphone wherein the insulating film that comprises the diaphragm is an electret. In alternate embodiments the foil element may be replaced by an element constructed of resilient foam-like material.

United States Patent 1191 Young, Jr. Jan. 22, 1974 [54] ELECTROSTATICTRANSDUCER HAVING 2,868,894 1/1959 Schultz 179/111 R RESILIENT ELECTRODEFOREIGN PATENTS OR APPLICATIONS Inventor: Robert oung, Jr., Nort fi 11-881,584 11/1961 Great B11 31 179/111 [73] Assignee: GTE AutomaticElectric Laboratories, Incorporated, Przmary Examiner-Kathleen H. ClaffyNorthlake, m Assistant Examiner--Thomas L. Kundert Attorney, Agent, orFirm- Robert .I. Black [22] Filed: Sept. 27, 1971 [21] Appl. No.:183,899 [57] ABSTRACT An electro-acoustic transducer having a flexibledia- [52] U.S. Cl 179/111 E, 179/111 R phragm n a kpl f ien n r on n a[51] Int. Cl [1041' 19/00 fr nt element of metalized foil. In apreferred embodi- [58] Field of Search..... 179/l ll R, 111 E, 180, 106merit of the invention the transducer is a condenser microphone whereinthe insulating film that comprises [56] References Cited the diaphragmis an electret. In alternate embodiments UNITED STATES PATENTS the foilelement may be replaced by an element ,con- 3 373 51 3/1968 Seder 1.79/11] R structed of resilient foam-like material. 3,646,280 2 1972 Tamuraet al 179/111 E 23 Claims, 3 Drawing Figures 2,645,301 7/1953 Vries179/180 PATENTEUJANZZISH 3.787,

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INVENTOR ROBERT F. YOUNG Jr.

AGENT ELECTROSTATIC TRANSDUCER HAVING RESILIENT ELECTRODE BACKGROUND OFTHE INVENTION 1. Field of the Invention The present invention pertainsto an electro-acoustic transducer having desirable electrical andacoustical properties. More specifically the present invention is drawnto a condenser microphone preferably of the electret type.

Electrostatic transducers and more particularly condenser microphonesare well known finding usage in almost any environment where microphonesare required. Condenser microphones are of particular interest because.of their simplicity of design and potential ease of manufacture. Untilrecently, however, condenser microphones required an associated directcurrent high voltage source to provide a necessary polarizing voltagefor the microphone.

This particular problem has been eliminated by usage of a prechargeddielectric material as an element of the condenser microphone. Theseelements are referred to as electrets.

Electrets are most easily defined in terms of measurements performed onthem. Given a material in block or slab form, one face is placed on aground plane and the potential is measured at the other face. The twofaces are interchanged and the process is repeated. If, withqualifications given below, the potentials differ, the

material is an electret;

There are two qualifications to the above. The first is that thepotential difference must not be due to static electricity. This may betaken care of in the same manner as is the same problem with phonographrecords. The second qualification is that the potential difference mustnot be due to domain phenomenon. Materials which exhibit domainphenomenon are called piezoelectrics, and are the true electricalanalogues of magnets. The determination-of whether or not a material ispiezoelectric is rather involved. I

There are three microscopic phenomena'which account for electretformation: dipole orientation, internal charge separation, and chargeinjection or removal. Certain materials, called polar, have asconstituents electric dipoles. These dipoles are usually orientedrandomly, creating no net effect. If these dipoles-are, at leastpartially, aligned in.the same direction, they no longer cancel eachothers effects, and a net result may be observed. Certain material s,called conductors, have as constituents charges ,which are more or lessfree to move within the material. These charges are usually arranged soas to cancel each others effects. If these charges are separated so thatthose of one polarity tend to be located near one face, and those of theopposite polarity tend to be located near the opposite face, then a netresult may be observed. Finally, if charges are injected into amaterial, or removed from it, in such a manner-as to create anunbalanced charge distribution between two opposite faces, a net effectmay be observed.

Any material, with the aforementioned exceptions, able to sustain thesephenomena for a usable period of time will, obviously, be an electret.Any process which produces any one or any combination of these effectsis called polarization, although only dipole orientation is properlycalled polarization. The merit of an electret is determined by twofactors: the strength of the polarization, and the rate of decay of thisstrength.

The known polarizing techniques all involve the placement of a materialbetween two electrodes of opposite polarity. These electrodes generate astrong electric field the direction of which is, of course, from thepositive electrode to the negative. In most polar materials the dipoleswill tend to be aligned in the direction of the field creating aneffective positive (negative) surface charge on the face nearest to thenegative (positive) electrode. In most conductive materials the positive(negative) charges will move towards the negative (positive) electrodecreating an effective positive (negative) surface charge on the facenearest to the negative (positive) electrode. Since, for these twoprocesses, the polarity of a face and a polarity of the electrodeclosest to that face are opposite, the surface charge generated by thesetwo processes is called heterocharge. Materials undergoingheterocharging are often heated because this increases dipole and chargemobility.

If the applied electric field is strong enough, charges may actually beinjected into or removed from the material. The face bythe positive(negative) electrode will receive positive (negative) charges or give upnegative (positive) charges. Since the lack of negative (positive)charge is equivalent to a surplus of positive (negative), injection andremoval lead to similar results. Since, for this process, the polarityof the face and the polarity of the electrode closest to that face arethe same, the surface charge generated by this phenomenon is calledhomocharge. The implementation of controlled charge injection issomewhat tricky since it involves the actual transport of material fromone body to another.

Both heterocharging and homocharging may occur simultaneously, and theresult is the difference of the magnitudes. Since the decay. rate foreach kind of charge may be different, an electret may actually undergocharge reversal. This would happen if the homocharge were initiallypredominent, but had a greater decay rate than the hetercharge. Adesirable situation would be one in which the decay rates were the samesince the difference, i.e. the net surface charge, would remain constanteven though both types of charge were decaying. Unfortunately, themechanism responsible for each type of charge is different, so it wouldbe only luck to find such material. Most people in the field now seem tobe looking for materials which have small decay rates for homocharge,and then use predominently charge injection. I

Electrets typically are composed of either organic or.

inorganic material. In the past organic substances such as beeswax orcarnuba wax were commonly used. However, because of their natural bulkonly relatively thick transducer elements could be reproduced, with theresult that close tolerances were difficult to maintain in thesesubstances. Fabrication of sufficiently thin diaphragms of such materialthat were sufficiently vibratile and yet possessedthe necessary mass andcompliance to yield high conversion efficiency were extremely difficultto form.

Certain plastic film materials however have shown excellent potentialfor the formation of thin film electrets. Such materials as Mylar andTeflon have been particularly outstanding inthis aspect.

2. Description of the Prior Art Successful electret transducers areshown in the prior U.S. Pats. to GM. Sessler et a1. Nos. 3,118,022 and3,118,979. Original attempts at thecreation of electrostatic transduceremploying electret elements, suggested the use of the electret film as avibratile diaphragm adapted to vibrate relative to a fixed rigidbackplate. Sessler interposes one or more additional thin dielectriclayers between the diaphragm and the backplate. As a result spacingbetween the diaphragm and the backplate is relatively large and thetransducer capacitance relatively low. Thus the transdcuer sensitivityis low because the electric field strength between diaphragm andbackplate is relatively low for the same bias voltage. In the Sesslerpatents the use of blind holes as well as dielectric layers between thediaphragm and backplate is taught. This arrangement is such that airbubbles entrapped therein do not communicate with the outside air,thereby preventing atmospheric pressure equalization. As a result airbubbles expand and contract with changes of temperature and atmosphericpressure thus changing the capacitance and sensitivity of thetransducer. In this manner the air filled holes of the prior transducerare entirely enclosed resulting in an acoustical impedance that is quitehigh and reduced only by the use of the multiplelayers whichmechanically align the bubbles in series. In another U.S. Pat. No.3,373,251 to CE. Seeler the use of a rigid porous backplate which ispartially or totally permeable to air is taught. In Seelers arrangement,a thin plastic film diaphragm is positioned directly upon a rigid porousbackplate touching at only certain distributed points dependent upon thesurface texture of the backplate. The metallic foil layer is placed onthe other side of the electret element to form a microphone. In theSeeler patent it is suggested that a suitably porous material for therigid backplate might be a sintered material consisting of a'pluralityof ball-shaped bronze powder elements compressed and heat-treated toprovide the desired, porosity. None of the designs taught by eitherSessler or Seeler are capable of providing the high compliance and highdamping characteris' tics that are found in the present invention.

SUMMARY OF THE INVENTION The electrostatic transducer disclosed in thepresent invention differs from previous electrostatic transducers andparticularly from prior art electrostatic transducers that includeelectret elements, by utilizing resilrated by a dielectric material, atleast one of the two electrically conductive elements according to thepresent invention is of resilient construction.

For example, the backplate may be of resilient electrically conductivefoam or similar material or alternately the front element placed infront of a dielectric diaphragm may be constructed of conductive foam.Or as would be obvious both backplate and front elements may be ofresilient construction. I

While the resiliently constructed elements may take several forms it hasbeen found highly desirable to utilize a plastic foam that includes ahigh percentage of carbon to render it conductive. Insuch a foammaterial the porosity can be controlled by the use of a blowing agent inthe fabrication of the foam itself. The degree of the resiliency of thefoam can be regulated through a choice of materials. As is obvious fromexamining plastic foam. foam rubber or similar materials the bulk of thevolume is comprised of air contained in pockets or cells. As aconsequence the surface texture is usually copious with deepinterstices. While the same texture may be had with solid materials, itis obvious they do not contain large amounts of air. I

A similar successful usage has also been made of cloth, the porosity ofwhich may be controlled by the weave and thread size. The resiliency maybe controlled by choice of materials. Obviously some form of metallicimpregnation or coating for conductivity is required. Whatever thechoice of material the output becomes a direct function of the surfacetexture and the damping or acoustical resistance is the direct functionof the porosity. Utilization of resilient elements provides a greaterrange of surface textures to choose from than might be available withtheuse of solid elements.-

The utilization of foam elements in front of the diaphragm providesexcellent damping particularly when the foam is in slight contact withthe diaphragm. The inclusion of both front and rear elements of foamprovides the obvious advantages of simplicity of electrical connectionand extreme shock resistance as well as maximum protection for thediaphragm material itself.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-section view of apreferred embodiment of an electrostatic transducer in accordance withthe present invention.

FIG. 2 is a cross-section view of an alternate arrangement of thepresent invention, 1

FIG. 3 is a cross-section view of another alternate arrangement of thepresent invention.

DESCRIPTION or THE PREFERRED EMBODIMENTS Referring now to FIG. 1 amicrophone embodying the principles of the present invention isshown.,The microphone consists of a housing 11 constructed of metal orother electrically conductive material. En-

closed within the housing is an electrically-conductive spacing orsupport ring 1-2 which is in direct contact withthe housing. Directlybeneath the support ring is the front element 13 of metallic foil orsimilar material. The front element 13 is in direct contact with thespacer ring 12 providing electrical circuit continuity. Directly belowfront element 13 and comprising a portion of the diaphragm is a layer ofplastic film such as Mylar, acting as a dielectric elementof the presentcondenser microphone. In a preferred embodiment the Mylar element 14 ispre-polarize'd to form an electret.

The manner in which the film layer 14 is prepolarized may include any ofthe known techniques of forming electrets, including the exposure of thematerial to corona discharge, heating in a high potential field, etc.

Supporting the diaphragm assembly at its periphery is an insulator ringthat extends around the inner circumference of the microhone housing.Alternately the insulator may be formed of several strips of dielectricmaterial spaced radially around the inner circumference of the housing.

The second element'or backplate of the microphone 16 is positioneddirectly below and in contact with the Mylar film portion of thediaphragm. In the present embodiment the ridges and points of thetopmost surface of the backplate 16 are in contact with and support theMylar film element 14. The backplate 16 is insulated from the microphonehousing by insulator l5 and is so spaced that it is not in contact withthe bottom portion of the housing 1 l. Alternately an additional layerof insulation between the backplate and the bottom of the housing may beprovided.

An electrical connection is made between the backplate 16 and terminal17A passing through the microphone housing and insulated from thehousing. A second electrical connection is made to the conductivehousing 11 and eiitended to terminal 173. At terminals 17A and 178connections are then made to an amplifier to be driven by the microphoneof the present invention.

As noted previously the backplate 16 is of resilient construction and inthe present embodiment is constructed of a plastic foammaterialimpregnated with carbon so as to make it electricallyconductive.

In operation sound waves entering the microphone cause the diaphragmassembly consisting of foil element l3 and the Mylar electret 14 tobe-forced toward the backplate 16, changing the relative capacitancebetween element 13 and backplate 16. The resultant capacitance changesare translated as electrical signals which are applied at terminals 17Aand 17B to an amplifier.

As an alternative the use of a film element 14, that has not beenpre-polarized is also possible. In this particular instance a directcurrent polarizing potential is applied across terminals 17A and 178 topolarize the capacitor that is formed by elements 13, backplate l6 anddielectric 14. Then by means of an audio coupling capacitor connected toeither terminal 17A or 178 the alternating current component resultingmay be transferred to an amplifier with return being made through theother terminal. a

The use of a resilient backplate in the present embodiment provides ahighly compliant acoustic impedance to sound waves directed toward themicrophone. The present backplate also provides a high damping qualityto the present unit. Different than prior art condenser microphones, noadditional impedance need be provided other than the backplateitself.The resilient quality of the backplate in addition to providing anacoustic impedance, provides a variable mechanical impedance helping torestore the diaphragm portion of the present transducer to its normalcondition.

Alternate arrangements of the present invention are shown in FIGS. 2 and3. In FIG. 2 the basic structure is similar to that shown in FIG. 1 withhousing 21, Mylar element 24. insulator 25, backplate 26 and terminals27A and 278 all corresponding to elements 11, 14, 15, 16, and 17A and17B respectively of FIG. 1. The principal difference in the embodimentshown in FIG. 2 being the replacement of the foil front element 13 ofFIG. 1 with a front element of'resilient construction 23. This elementmay be of construction similar to that of the backplate 26, i.e., aresiliently constructed element of plastic foam impregnated with carbonor similar material to make it electrically conductive.

As may be observed by referring to FIG. 2 the front element 23 is incontact with the electrically conductive case 21 at its sides and aportion of its top surface, with the bottom of element 23 having pointsand ridges in contact with the upper surface of the Mylar film 24.Operationally this embodiment is similar to that described previouslywhile providing greater shock resistance and protection againstdiaphragm rupture. Increased efficiency of damping is also provided byutilization of the resilient foam element 23 which is in slight contactwith the Mylar element 24.

Yet another arrangement is shown in FIG. 3 wherein elements 31, 33, 34,35, 36, 37A and 378 all correspond respectively to similar elements 21,23, 24, 25, 26 and 27A and 27B of FIG. 2. The principal differencebetween the arrangement shown in FIG. 3 and that of FIG. 2 is theinclusion of additional dielectric spacers or strips 38 and 39 with thestrips 38 placedbetween the lower surface of front element 33 and theupper surface of Mylar diaphragm 34 and strips 39 providing a spacingbetween the lower surface of Mylar diaphragm 34 and the upper surface ofbackplate 36. In this particular embodiment the additional air spaceabove and below the diaphragm acts to improve the vibratile action ofthe diaphragm. The inclusion of the spacing strips 38 and 39 whileslightly decreasing the capacitance of the present unit acts to increasethe acoustic efficiency.

Numerous other embodiments incorporating the resilient elements of thepresent invention can be constructed utilizing some or all of thetechniques described above. For example, the use of resilient frontelements and solid backplates or elements providing spacers associatedwith only front element or backplate would be well within the teachingsof the present invention.

Likewise all of the techniques disclosed may be applied to transducerforms, other than microphones. The principles disclosed are equally wellapplied to earphones, loudspeakers, etc. Numerous other modificationswould also be obvious to one skilled in the art and accordingly thepresent invention should only be limited by scope of the claims appendedhereto.

What is claimed is: i

1. An electrostatic transducer comprising: a movable diaphragm ofdielectric material including first and second surface areas; a firstelectrically conductive element positioned adjacent to said diaphragmfirst surface area; a'second electrically conductive element positionedadjacent said diaphragm second surface area; at least one of saidelectrically conductive elements being movable and of resilientconstruction, said resilient construction enabling said conductiveelement to function as a variable mechanical impedance; and electricalconductors connected to said elements, to permit electrical connectionsto be made to said transducer.

2. An electrostatic transducer as claimed in claim 1 wherein saiddiaphragm dielectric material is prepolarized.

3. An electrostatic transduceras claimed in claim 1 wherein saiddiaphragm dielectric material is polarized by application of a biasvoltage to said electrical conductors.

4. An electrostatic transducer as claimed in claim 1 wherein said secondelement is movable and of resilient construction.

5. An electrostatic transducer as claimed in claim 4 wherein said firstelement is constructed of electrically conductive foil.

6. An electrostatic transducer as claimed in claim wherein said foilfirst element is in direct contact with said diaphragm first surfacearea.

7. An electrostatic transducer as claimed in claim 4 wherein saidresiliently constructed second element includes a plurality ofraisedareas in contact with said diaphragm second surface area.

8. An electrostatic transducer as claimed in claim 4 wherein saidresiliently constructed second element is spatially separated from saiddiaphragm second surface area, by spacing means.

9. An electrostatic transducer as claimed in claim 8 wherein saidspacing means comprise a plurality of strips of dielectric materialpositioned between said second element and said diaphragm second'surfacearea.

10. An electrostatic transducer as claimed in claim 4 wherein saidresiliently constructed second element is formed of electricallyconductive foam material.

11. An electrostatic transducer as claimed in claim 10 wherein said foammaterial contains a substantially high percentage of carbon.

12. An electrostatic transducer as claimed in claim 1 wherein said firstelectrically conductive element is movable and of resilientconstruction.

13. An electrostatic transducer as claimed in claim 12 wherein saidresiliently constructed first element in cludes a plurality of raisedareas in contact with said diaphragm first surface area. 7

14. An electrostatic transducer as claimed in claim 12 wherein saidresiliently constructed first element is spatially separated from saiddiaphragm first surface area, by spacing means. a

15. An electrostatic transducer as claimed in claim 14 wherein saidspring means comprise a plurality of strips of dielectric materialpositioned between said first element and said diaphragm first surfacearea.

16. An electrostatic transducer as claimed in claim 12 wherein saidresiliently constructed first element is formed of electricallyconductive foam material.

17. An electrostatic transducer as claimed in claim 16 wherein said foammaterial contains a substantially high percentage of carbon.

18. An electrostatic transducer as claimed in claim 1 wherein said firstand second electrically conductive elements are both movable and ofresilient construction.

19. An electrostatic transducer as claimed in claim 18 wherein saidresiliently constructed first element includes a plurality of raisedareas in contact with said diaphragm first surface area and saidresiliently constructed second element includes a plurality of raisedareas in contact with said diaphragm second surface area.

20. An electrostatic transducer as claimed in claim 18 wherein saidresiliently constructed first element is spatially, separated from saiddiaphragm first surface area by first spacing means and said resilientlyconstructed second element is spatially separated from said diaphragmsecond surface area by second spacing means. a

21. An electrostatic transducer as claimed in claim 20 wherein saidfirst spacing means comprise a plurality of strips of dielectricmaterial positioned between said first element and said diaphragm firstsurface area and said second spacing means comprise a plurality ofstrips of dielectric material positioned between said second element andsaid diaphragm second surface area.

22. An electrostatic transducer as claimed in claim 18 wherein saidresiliently constructed first and second high percentage of carbon.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. DatedJanuary 22, 1974 Inven Robert F. Young Jr.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 7, line 35, the word "spring" should read --spacing- Signed andsealed this 21st day of May 1974.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. v C. MARSHALL DANN Attesting Officer Commissionerof Patents FORM PO-1050 (10-69) USCOMM-DC 60376-P69 W U.S, GOVERNMENTPRINTING OFFICE: IQII 0-366-3JA.

1. An electrostatic transducer comprising: a movable diaphragm of dielectric material including first and second surface areas; a first electrically conductive element positioned adjacent to said diaphragm first surface area; a second electrically conductive element positioned adjacent said diaphragm second surface area; at least one of said electrically conductive elements being movable and of resilient construction, said resilient construction enabling said conductive element to function as a variable mechanical impedance; and electrical conductors connected to said elements, to permit electrical connections to be made to said transducer.
 2. An electrostatic transducer as claimed in claim 1 wherein said diaphragm dielectric material is pre-polarized.
 3. An electrostatic transducer as claimed in claim 1 wherein said diaphragm dielectric material is polarized by application of a bias voltage to said electrical conductors.
 4. An electrostatic transducer as claimed in claim 1 wherein said second element is movable and of resilient construction.
 5. An electrostatic transducer as claimed in claim 4 wherein said first element is constructed of electrically conductive foil.
 6. An electrostatic transducer as claimed in claim 5 wherein said foil first element is in direct contact with said diaphragm first surface area.
 7. An electrostatic transducer as claimed in claim 4 wherein said resiliently constructed second element includes a plurality of raised areas in contact with said diaphragm second surface area.
 8. An electrostatic transducer as claimed in claim 4 wherein said resiliently constructed second element is spatially separated from said diaphragm second surface area, by spacing means.
 9. An electrostatic transducer as claimed in claim 8 wherein said spacing means comprise a plurality of strips of dielectric material positioned between said second element and said diaphragm second surface area.
 10. An electrostatic transducer as claimed in claim 4 wherein said resiliently constructed second element is formed of electrically conductive foam material.
 11. An electrostatic transducer as claimed in claim 10 wherein said foam material contains a substantially high percentage of carbon.
 12. An electrostatic transducer as claimed in claim 1 wherein said first electrically conductive element is movable and of resilient construction.
 13. An electrostatic transducer as claimed in claim 12 wherein said resiliently constructed first element includes a plurality of raised areas in contact with said diaphragm first surface area.
 14. An electrostatic transducer as claimed in claim 12 wherein said resiliently constructed first element is spatially separated from said diaphragm first surface area, by spacing means.
 15. An electrostatic transducer as claimed in claim 14 wherein said spring means comprise a plurality of strips of dielectric material positioned between said first element and said diaphragm first surface area.
 16. An electrostatic transducer as claimed in claim 12 wherein said resiliently constructed first element is formed of electrically conductive foam material.
 17. An electrostatic transducer as claimed in claim 16 wherein said foam material contains a substantially high percentage of carbon.
 18. An electrostatic transducer as claimed in claim 1 wherein said first and second electrically conductive elements are both movable and of resilient construction.
 19. An electrostatic transducer as claimed in claim 18 wherein said resiliently constructed first element includes a plurality of raised areas in contact with said diaphragm first surface area and said resiliently constructed second element includes a plurality of raised areas in contact with said diaphragm second surface area.
 20. An electrostatic transducer as claimed in claim 18 wherein said resiliently constructed first element is spatially separated from said diaphragm first surface area by first spacing means and said rEsiliently constructed second element is spatially separated from said diaphragm second surface area by second spacing means.
 21. An electrostatic transducer as claimed in claim 20 wherein said first spacing means comprise a plurality of strips of dielectric material positioned between said first element and said diaphragm first surface area and said second spacing means comprise a plurality of strips of dielectric material positioned between said second element and said diaphragm second surface area.
 22. An electrostatic transducer as claimed in claim 18 wherein said resiliently constructed first and second elements are both formed of electrically conductive foam material.
 23. An electrostatic transducer as claimed in claim 22 wherein said foam material contains a substantially high percentage of carbon. 